Synchronous amplitude modulation for improved performance of optical transmission systems

ABSTRACT

A method and apparatus is provided that yields improved performance of both single channel and WDM long-distance optical transmission systems by synchronously modulating of the transmitted signal&#39;s amplitude. An amplitude modulator receives an optical signal onto which data has been modulated at a predetermined frequency. The modulator re-modulates the amplitude of the optical signal in a continues fashion with a waveform that is periodic, whose fundamental frequency is equal to the same predetermined frequency at which the data is modulated onto the optical signal. The resulting signal (which is neither a pure NRZ or RZ signal) is more tolerant to the distortions usually found in lightwave transmission systems, thus giving superior transmission performance.

FIELD OF THE INVENTION

[0001] The invention relates to the optical transmission of informationand more particularly, to a method and apparatus for improvingtransmission capabilities over optical fiber transmission systems.

BACKGROUND OF THE INVENTION

[0002] Very long optical fiber transmission paths, such as thoseemployed in undersea or transcontinental terrestrial lightwavetransmission systems, which employ optical amplifier repeaters, aresubject to decreased performance due to a host of impairments thataccumulate along the length of the optical fiber comprising thetransmission path. The source of these impairments within a single datachannel include amplified spontaneous emission (ASE) noise generated inthe Erbium-Doped Fiber-Amplifiers (EDFAs), nonlinear effects caused bydependence of the single-mode fiber's index on the intensity of thelight propagating through it, and chromatic dispersion which causesdifferent optical frequencies to travel at different group velocities.In addition, for wavelength division multiplexed (WDM) systems, whereseveral optical channels might be on the same fiber, crosstalk betweenchannels caused by the fiber's nonlinear index must be considered.Typically, it is advantageous to operate long-haul transmission systemsat high data rates per channel. For example, multiples of theSynchronous Digital Hierarchy (SDH) standard of 2.5 Gb/s are generallyconsidered useful. Generally speaking, the impairments that limit thesystem's performance cause two types of degradations in the received eyepattern, which are related to randomness (caused by noise) anddeterministic degradations (or distortions in the received bit pattern).Distortions of the second type are sometimes referred to as Inter-SymbolInterference (ISI). As the bit rates rise into the gigabit per secondrange it becomes critical to manage those impairments that effect theshape of the received pulses, and to limit the ISI.

[0003] Distortions of the received waveform are influenced by the shapeof the transmitted pulses and the details of the design of thetransmission line. Two signaling formats considered useful in long-haultransmission systems are the non-return-to-zero (NRZ) and solitonsformats. The transmission format used in most long-haul lightwave systemis the NRZ format because it is easy to generate, detect and process.The name NRZ is applied to this format because it describes thewaveform's constant value characteristic when consecutive binary onesare sent. Alternatively, a string of binary data with optical pulsesthat do not occupy the entire bit period are described generically asReturn-to-Zero or RZ. The two most common examples of RZ signalingpulses are a rectangular pulse that occupies one half of the bit period,and a hyperbolic secant pulse (or soliton) with a pulse width of about ⅕of the time slot.

[0004] Known methods of reducing noise and distortion in lightwavetransmission systems include the application of synchronous polarizationand phase modulation to the NRZ signaling format (see U.S. Pat. No.5,526,162), dispersion management of the transmission line, or the useof optical solitons. Scrambling the state-of-polarization of the opticalcarrier at the bit-rate of the transmitted NRZ signal can greatlyimprove the transmission performance of long-haul optical amplifiedtransmission systems. In addition to synchronous polarizationscrambling, superimposed phase modulation (PM) can dramatically increasethe eye opening of the received data pattern. This increase results fromthe conversion of PM into bit-synchronous amplitude modulation (AM)through chromatic dispersion and nonlinear effects in the fiber. Thesesynchronous polarization/phase modulations techniques were used in a WDMtransmission system having a total transmission capacity of 100 Gb/s (20WDM channels at 5 Gb/s) over 6300 km, as discussed in Bergano, et al.,“100 Gb/s WDM Transmission of Twenty 5 Gb/s NRZ Data Channels OverTransoceanic Distances Using a Gain Flattened Amplifier Chain,” EuropeanConference on Optical Communication (ECOC'95), Paper Th.A.3.1, Brussels,Belgium, Sep. 17-21, 1995.

[0005] While these methods have been effective, it is desirable tofurther reduce distortion to improve the performance of long distanceoptical transmission systems.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, a method and apparatusis provided that yields improved performance of both single channel andWDM long-distance optical transmission systems by synchronouslymodulating of the transmitted signal's amplitude. An amplitude modulatorreceives an optical signal onto which data has been modulated at apredetermined frequency. The modulator re-modulates the amplitude of theoptical signal in a continues fashion with a waveform that is periodic,whose fundamental frequency is equal to the same predetermined frequencyat which the data is modulated onto the optical signal. The resultingsignal (which is neither a pure NRZ or RZ signal) is more tolerant tothe distortions usually found in lightwave transmission systems, thusgiving superior transmission performance.

[0007] In accordance with one aspect of the invention, an opticaltransmission system is provided that includes a transmitter, an opticaltransmission path coupled to the transmitter, and a receiver coupled tothe optical transmission path. The transmitter includes an opticalsignal source for generating an optical signal onto which data ismodulated at a predetermined frequency. An amplitude modulator iscoupled to the optical signal source for modulating the intensity of thedata modulated signal. A clock, which is coupled to the amplitudemodulator, has a frequency that determines the frequency of theamplitude modulator. The frequency of the clock is phase locked andequal to the predetermined frequency at which data is modulated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows a simplified block diagram of one embodiment of thesynchronous amplitude modulated transmitter in accordance with thepresent invention.

[0009]FIG. 2 shows typical waveforms and their corresponding eyediagrams produced by the transmitter shown in FIG. 1

[0010]FIG. 3 shows typical eye diagrams produced by the transmitter inFIG. 1 for differing periods of delay between the data modulation andthe synchronous amplitude modulation.

[0011]FIG. 4 shows a simplified block diagram of an alternativeembodiment of the transmitter that includes synchronous optical phasemodulation, amplitude modulation, and polarization modulation.

[0012]FIG. 5 shows the resulting Q-factor verses the level ofsynchronous amplitude modulation for an arrangement similar to thatshown in FIG. 4.

[0013]FIG. 6 shows an embodiment of a transmission system architecturein accordance with the present invention.

DETAILED DESCRIPTION

[0014]FIG. 1 shows a simplified block diagram of an exemplary opticaltransmitter facilitating the practice of the invention. As shown, theinvention includes a laser 100 for producing a continuous wave (CW)optical signal 101. The optical signal 101 is transmitted to a datamodulator 102 that modulates the signal to impart information thereto ina well known fashion, producing a modulated optical information signal103. The data modulator 102 receives the data to be imparted to theoptical signal 101 from a data source 104 and modulates the opticalsignal 101 at a frequency determined by a clock 106. The opticalinformation signal 103 is transmitted from the data modulator 102 to anamplitude modulator 107 which places additional intensity modulation onthe optical information signal 103. Modulators 102 and 107 could be, forexample, a 10 Gb/s modulator manufactured by Lucent Technologies asmodel number 2023.

[0015] In accordance with the present invention, the amplitude modulator107 is driven by the clock 106 so that the intensity of the opticalinformation signal 103 is re-modulated at a rate equal to the rate atwhich data is imparted to the optical signal 101, which is defined byclock 106. As further shown in FIG. 1, it may be advantageous to providean electrical variable-delay 109 and an amplitude adjustment mechanism110 which couple the clock 106 to the amplitude modulator 107. Thevariable delay 109 may be employed to selectively adjust the phase ofthe amplitude modulation imparted by amplitude modulator 107 relative tothe phase of the data modulation imparted by data modulator 102. Theamplitude adjustment mechanism 110 is employed to adjust the modulationdepth that amplitude modulator 107 imparts to optical information signal103. The optimal settings for these adjustments will depend on manyparameters and can be determined empirically. Moreover, in a WDM system,the optimal setting for each channel would not necessarily be the sameand thus the channels may be individually optimized.

[0016] The manner in which the clock 106 drives the amplitude modulator107 may be described by examining the electric field components of theoptical signal 103 on which the amplitude modulator acts. In x-ycoordinates these components may be expressed as follows:

E _(x)(t)=A _(x)(t)e ^(i(ωt+φ) ^(_(x)) ^((t)))  (1)

E _(y)(t)=A _(y)(t)e ^(t(ωt+φ) ^(_(y)) ^((t)))  (2)

[0017] where ω is the optical carrier frequency, A_(x)(t) and A_(y)(t)are assumed to be real field amplitudes which include the intensitymodulation imposed by data modulator 102, and φ_(x)(t) and φ_(y)(t) arethe optical phase components and include any optical phase modulationthat might be present. The amplitude modulator 107 serves to modulatethe optical signal by varying only the real amplitudes A_(x)(t) andA_(y)(t), with a function F(t) that is periodic and has a fundamentalfrequency component that is equal to, and phase locked to, the clocksignal generated by clock 106. Amplitude modulator 107 impresses anadditional amplitude modulation such that the intensity of opticalinformation signal 103 is multiplied by I(t). For purposes ofillustration it is assumed that the periodic function F(t) is normalizedto be in the range bounded by [+1,−1]. I(t) is given by;

I(t)=0.5*[(1−B)F(t+Ψ _(am))+1+B]  (3)

[0018] $\begin{matrix}{B \equiv {\frac{100 - A_{am}}{100 + A_{am}}\quad 0} \leq A_{am} \leq 100} & (4)\end{matrix}$

[0019] where A_(am) is the percentage of amplitude modulation placed onoptical information signal 103 by modulator 107, and Ψ_(am) is the phaseangle of the modulation with respect to the data modulation. Thus, I(t)is simply a scaled version of the periodic function F(t) with a maximumvalue of unity, a minimum value of B, and is offset in time by Ψ_(am).It is anticipated that the level of the amplitude modulation is adjustedby amplitude adjustment mechanism 110, and the offset Ψ_(am) is adjustedby variable delay 109. The signal 108 from the transmitter is thenrepresented by the following electric field components:

E _(x-out)(t)={square root}{square root over (I(t))}A _(x)(t)e ^(t(ωt+φ)^(_(x)) ^((t)))  (5)

E _(y-out)(t)={square root}{square root over (I(t))}A _(y)(t)e ^(i(ωt+φ)^(_(y)) ^((t)))  (6)

[0020] Equations (5) and (6) have been written in general terms for anyperiodic function that fits the above description. However, it may beadvantageous to employ sinusoidal modulation, which will be the basisfor the illustrative waveforms shown in FIG. 2 and FIG. 3.

[0021]FIG. 2 shows a series of typical waveforms representing outputsignal 108 when the periodic waveform providing the additional amplitudemodulation is a sinusoidal function. Each waveform, which comprisestwelve bits, results from a different level of modulation depth impartedby the amplitude modulator 107. Adjacent each waveform is itscorresponding eye diagram. Waveform 201 and its corresponding eyediagram 202 are typical examples of a conventional NRZ waveform.Waveforms 203, 205, 207, 209, and 211, which respectively correspond toeye diagrams 204, 206, 208, 210, and 212, show waveforms for amplitudemodulation levels of 20%, 40%, 60%, 80%, and 90%, respectively.

[0022] The waveforms generated by the present invention do notconveniently fit the definition of any conventional modulation format.For example, the waveforms shown in FIG. 2 are not constant in valueover contiguous “1” bits and thus do not fit the standard definition ofthe NRZ format. In addition, since the waveforms do not necessarilyreturn to zero between adjacent bits, they do not fit the standarddefinition of the RZ format. This waveform generated by the presentinvention provides a tradeoff between two regimes in the transmissionsystem. The energy in the pulses is more concentrated near the center ofthe bit slot, which is desirable for limiting the amount of ISI, butsince the bit almost fills the bit slot, the peak intensity is not aslarge as it would be, for example, in a soliton system. In addition, therise and fall times of the pulses are reduced, which has the potentialbenefit of lowering the amount of chirp induced on the pulse by thefiber's nonlinear index.

[0023] One of ordinary skill in the art will recognize that thewaveforms shown in FIG. 2 may be produced by variants of the transmittershown in FIG. 1. For example, the modulation imparted in FIG. 1 by theamplitude modulator could be alternatively generated by electrical meansprior to impressing the optical carrier signal 101 with data. Forexample, the data source 104 could supply to the modulator 102electrical waveforms similar to those in shown in FIG. 2 so that theamplitude modulation is directly imparted onto the carrier signal 101.Alternatively, such an electrical waveform could be used to directlymodulate a semiconductor laser, such as a distributed feedback laser.Also it is appreciated that the periodic waveform used to drive theadditional modulation stage 107 in FIG. 1 need not be a sinusoid.

[0024]FIG. 3 illustrates the effects of the delay element 109 on the eyediagram by showing five eye diagrams of the output signal 108 fordifferent phase offsets. The eye diagrams in this figure were allproduced using a sinusoidal amplitude modulation level of 60%, similarto eye diagram 208 in FIG. 2. In eye diagram 301 the phase of theamplitude modulation is aligned with the phase of the impressed data.Eye diagrams 302 and 303 were produced by shifting the phase of theamplitude modulation by −30° and −60°, respectively, with respect to theimpressed data. Similarly, eye diagrams 304 and 305 were produced byshifting the phase of the amplitude modulation by +30° and +60°,respectively, with respect to the impressed data. A known amount of skewcan be conveniently built into the transmitted eye by shifting themodulation phase in this manner. This feature could be used to correctfor certain impairments found in high-speed lightwave communications.For example in systems using 10 Gb/s carriers, it is known that thesingle-mode fiber's third order dispersion can cause a skew in thereceived eye. By placing a known amount of skew in the transmitted eyeit may be possible to offset some of the penalty associated with theimpairment caused by the known waveform distortions.

[0025]FIG. 4 shows an alternative embodiment of the invention in whichthe additional amplitude modulator 107 is used in connection with atransmitter employing synchronous polarization and optical phasemodulation. An example of such a transmitter is disclosed in U.S. Pat.No 5,526,162 to Bergano. In FIG. 4, a laser 400 produces a continuouswave (CW) optical signal 401. The optical signal 401 is transmitted to adata modulator 402 that modulates the signal to impart informationthereto in a well known fashion, producing a modulated opticalinformation signal 403. The data modulator 402 receives the data to beimparted to the optical signal 401 from a data source 404 and modulatesthe optical signal 401 at a frequency determined by a clock 405. Theoptical information signal 403 is transmitted from the data modulator402 to optical phase modulator 406, amplitude modulator 407, and finallyto polarization modulator 413. The clock 405 drives the three modulationstages via a series of variable delay elements 408, 409, and 414, whichare used to selectively adjust the delay of the modulation imparted bymodulators 406, 407, and 413 relative to the phase of the datamodulation imparted by modulator 402. In accordance with the presentinvention, the amplitude modulator 407 is driven by the clock 405 sothat the intensity of the optical information signal is re-modulated ata rate equal to the rate at which data is imparted to the optical signal401. Similar to the FIG. 1 embodiment, an amplitude adjustment mechanism410 is employed to set the modulation depth that amplitude modulator 410imparts on signal 413.

[0026] The manner in which the clock 405 drives phase modulator 406,amplitude modulator 407, and polarization modulator 413 may be describedby examining the electric field components of the optical signal 415.These components are similar to those presented in equations (5) and (6)with the inclusion of additional phase terms. For example, assume thatthe synchronous modulation imparted by the modulators is sinusoidal. Thetransmitter shown in FIG. 4 modifies the optical phase of the signalproduced by the transmitter of FIG. 1 while the amplitude is unchanged.In this case the phase modulation imparted to the optical signalincludes two separate and independent phases: a phase Ψ₂ associated withpolarization modulator 413 and a phase Ψ₁ associated with the opticalphase modulator 406. Thus, the phase angles φ_(x) and φ_(y) of theoptical signal 415 launched from the polarization modulator become:

φ_(x)(t)=a _(x) cos(Ωt+Ψ ₂)+b cos(Ωt+Ψ ₁)  (7)

φ_(y)(t)=a _(y) cos(Ωt+Ψ ₂)+b cos(Ωt+Ψ ₁)  (8)

[0027] where a_(x) and a_(y) are the phase modulation indices of thepolarization modulator, b is the phase modulation index of the opticalphase modulator, Ψ_(1,2) are the phase offsets set by delay elements 408and 414, respectively, and Ω is the bitrate determined by clock 405.

[0028] As equations (7) and (8) indicate, the optical phase modulator406 imparts the same phase modulation to both the x and y components ofthe optical signal. Accordingly, the optical phase modulator 406modulates the optical phase of signal 403 without modulating itspolarization. The reason the optical phase modulator 406 does notmodulate the polarization is because the polarization modulation of theoptical signal is proportional to the difference between the phasesφ_(x) and φ_(y) and this difference is unaffected by the optical phasemodulator 406 since it modulates both φ_(x) and φ_(y) by equal amounts.In principle, every possible State-of-Polarization (SOP) of amonochromatic signal having these electric field components can beobtained by varying the ratio a_(x)/a_(y) while maintaining the value of(a_(x) ²+a_(y) ²) constant and varying the relative phase differenceφ_(x)−φ_(y) between 0 and 2π. However, the polarization modulator 413serves to modulate the SOP of the optical signal by varying only thedifference of the phases φ_(x) and φ_(y), which is sufficient to providea SOP whose average value over a modulation cycle is low. Polarizationmodulator 413 alters the SOP of the optical information signal in such away that the degree of polarization over the modulation period isreduced from unity. Accordingly, the output signal 415 has a degree ofpolarization that can be substantially equal to zero and is said to bepolarization scrambled. The polarization modulator 413 may serve totrace the SOP of optical information signal 415 on a complete greatcircle of the Poincarésphere. Alternatively, the SOP of the opticalsignal may reciprocate along the Poincarésphere. In either case, theaverage value of the SOP over each modulation cycle is substantiallylowered from its normal value of unity.

[0029] One of ordinary skill in the art will recognize that thefunctions of the various modulators are shown in FIG. 4 for purposes ofillustration only and that two or more of the modulators may be realizedin a single functional unit. For example, as previously mentioned, datamodulator 402 may also function as the amplitude modulator 407 by havingthe data source 404 provide the proper electrical drive signal. Inaddition, the functions of phase modulator 406 and polarizationmodulator 413 may be combined in a manner similar to that shown in FIG.3 of U.S. Pat. No. 5,526,162.

[0030] The experimental results presented in FIG. 5 were obtained from atransmitter of the type shown in FIG. 4, which incorporated an NRZtransmitter having synchronous amplitude, phase, and polarizationmodulation. The transmission path, which used circulating looptechniques, extended 9,300 kms and employed twenty WDM channels, eachoperating at a bit rate of 5.0 Gbits/sec with an average launch power of+7 dBm for all of the channels. The experiment was similar to theresults for a twenty channel system presented by Bergano and Davidson inIEEE Journal of Lightwave Technology, Vol. 14, No. 6, p. 1299 June 1996,except that in the present arrangement the EDFAs were pumped at 980 nm,which improved the noise figure and increased the transmission distance.FIG. 5 shows the resulting Q-factor (i.e., the electricalsignal-to-noise ratio) versus the depth of modulation for channels 3 and19. The two channels are representative of two different chromaticdispersion regimes of the system. Channel 3, located at 6.8 nm below thezero dispersion wavelength λ₀, had an average dispersion of −0.51ps/km-nm and channel 19, located 2.8 nm above the zero dispersionwavelength λ₀, had an average dispersion of +0.21 ps/km-nm. The dataindicates that good Q-factor performance can be achieved by selecting anappropriate value for the depth of modulation. The appropriate valuediffers from both the pure NRZ format (0% depth of modulation) and theRZ format (greater than 100% depth of modulation). FIG. 5 also providesa definition used to calculate the depth of modulation.

[0031]FIG. 6 is an example of a transmission system including atransmitter, receiver, transmission path, and telemetry path inaccordance with the present invention. Shown are a synchronouslymodulated transmitter 601 such as shown in FIGS. 1 or 4, transmissionmedium 602, and telemetry path 603 which connects equipment at thereceiver side to the transmitter side to feedback a characteristic ofthe received signal such as the Q-factor. Transmission medium 602, forpurposes of this example, but not as a limitation on the invention, is achain of optical amplifiers and single-mode optical fibers. Theseelements are well known in the art. Transmitter 601 produces an opticalinformation signal whose amplitude, and/or optical phase andpolarization is synchronously modulated as described above. At thereceiver, the Q-factor is measured as an indication of transmissionperformance with a Q-factor measurement apparatus 605. The Q-factor,which provides a method for determining the transmission performance ofsignals after propagation through lightwave systems, is discussed inBergano et al., IEEE Phot. Tech. Lett., Vol. 5, No. 3, March 1993.Apparatus 605 could be, for example, a Q measurement unit manufacturedby Advantest under the model number D3281. The Q-factor is sent back tothe transmitter 601 via telemetry path 603. It will be appreciated bythose skilled in the art that it may be desirable, in some applications,for telemetry path 603 to be part of the same transmission system, suchas overhead bits in a SDH frame, or an order-wire channel, or betransmitted on a different channel, such as a separate phone line. TheQ-factor is received and processed by a logic element that may belocated, for example, within the synchronously modulated transmitter601. The logic element controls the level and the relative timing of thevarious modulation stages imparted to the signal 60 to maximize thereceived Q-factor. This type of feedback system could assist inmaintaining adequate transmission performance in the presence of afading channel, which can be caused by polarization effects.

1. An apparatus for transmitting an optical signal comprising: anoptical signal source for generating an optical signal onto which datais modulated at a predetermined frequency; an amplitude modulatorcoupled to the optical signal source for modulating the intensity ofsaid data modulated signal; and a clock coupled to the amplitudemodulator having a frequency that determines the modulation frequency ofthe amplitude modulator, said frequency of the clock being phase lockedand equal to said predetermined frequency.
 2. The apparatus of claim 1wherein the optical signal source includes a continuous-wave opticalsignal generator and a data source, said clock being coupled to the datasource for establishing the predetermined frequency at which data ismodulated onto the optical signal.
 3. The apparatus of claim 1 whereinthe amplitude modulator modulates the amplitude of the data modulatedoptical signal at said predetermined frequency with a prescribed phase,and further comprising an electrical variable-delay line coupling saidclock to said amplitude modulator for selectively varying the prescribedphase.
 4. The apparatus of claim 3 wherein said electricalvariable-delay line is a phase shifter.
 5. The apparatus of claim 1wherein said amplitude modulator includes means for selectivelyadjusting the degree of amplitude modulation that is imparted to saiddata modulated signal.
 6. The apparatus of claim 1 further comprising apolarization modulator coupled to said amplitude modulator and saidclock for modulating the state of polarization of said data modulatedsignal at said predetermined frequency such that an average value of thestate of polarization over a modulation cycle is substantially equal tozero.
 7. The apparatus of claim 6 wherein said polarization modulatormodulates the state of polarization by tracing the polarization of saidoptical signal along at least a portion of a Poincaré sphere.
 8. Theapparatus of claim 6 wherein the polarization modulator modulates thestate of polarization of the optical signal at said predeterminedfrequency with a prescribed phase, and further comprising an electricalvariable-delay line coupling said clock to said polarization modulatorfor selectively varying the prescribed phase.
 9. The apparatus of claim8 wherein said electrical variable-delay line is a phase shifter. 10.The apparatus of claim 6 further comprising an optical phase modulatorcoupling the optical signal source to the amplitude modulator, saidoptical phase modulator providing optical phase modulation to said datamodulated signal while imparting substantially no polarizationmodulation thereto.
 11. The apparatus of claim 10 wherein said clock iscoupled to said optical phase modulator so that said optical phasemodulator provides optical phase modulation at a frequency that is phaselocked and equal to said predetermined frequency.
 12. The apparatus ofclaim 11 further comprising a second electrical variable-delay linecoupling said clock to said optical phase modulator for selectivelyvarying the phase of said optical phase modulation provided by theoptical phase modulator.
 13. The apparatus of claim 12 wherein saidsecond electrical variable-delay line is a phase shifter.
 14. Anapparatus for transmitting an optical signal comprising: an amplitudemodulator receiving an optical signal onto which data has been modulatedat a predetermined frequency; and a clock coupled to the amplitudemodulator having a frequency that determines the frequency of themodulation cycle, said frequency of the clock being phase locked andequal to said predetermined frequency.
 15. The apparatus of claim 14wherein the amplitude modulator modulates the amplitude of the opticalsignal at said predetermined frequency with a prescribed phase, andfurther comprising an electrical variable-delay line coupling said clockto said amplitude modulator for selectively varying the prescribedphase.
 16. The apparatus of claim 15 wherein said electricalvariable-delay line is a phase shifter.
 17. The apparatus of claim 14further comprising a polarization modulator coupled to said amplitudemodulator and said clock for modulating the state of polarization ofsaid data modulated signal at said predetermined frequency such that anaverage value of the state of polarization over a modulation cycle issubstantially equal to zero.
 18. The apparatus of claim 17 wherein saidpolarization modulator modulates the state of polarization by tracingthe polarization of said optical signal along at least a portion of aPoincaré sphere.
 19. The apparatus of claim 18 wherein the polarizationmodulator modulates the state of polarization of the optical signal atsaid predetermined frequency with a prescribed phase, and furthercomprising an electrical variable-delay line coupling said clock to saidpolarization modulator for selectively varying the prescribed phase. 20.The apparatus of claim 19 wherein said electrical variable-delay line isa phase shifter.
 21. The apparatus of claim 17 further comprising anoptical phase modulator coupling the optical signal source to theamplitude modulator, said optical phase modulator providing opticalphase modulation to said data modulated signal while impartingsubstantially no polarization modulation thereto.
 22. The apparatus ofclaim 21 wherein said clock is coupled to said optical phase modulatorso that said optical phase modulator provides optical phase modulationat a frequency that is phase locked and equal to said predeterminedfrequency.
 23. The apparatus of claim 22 further comprising a secondelectrical variable-delay line coupling said clock to said optical phasemodulator for selectively varying the phase of said optical phasemodulation provided by the optical phase modulator.
 24. The apparatus ofclaim 23 wherein said second electrical variable-delay line is a phaseshifter.
 25. A method for transmitting an optical signal comprising thesteps of: generating an optical signal onto which data is modulated at apredetermined frequency; and modulating the amplitude of said datamodulated signal at a frequency phase locked and equal to saidpredetermined frequency.
 26. The method of claim 25 further comprisingthe step of selectively varying the phase of the amplitude modulationimparted to said data modulated signal.
 27. The method of claim 25further comprising the step of selectively phase modulating said datamodulated signal while imparting substantially no polarizationmodulation to the optical signal.
 28. The method of claim 27 wherein thestep of selectively phase modulating said data modulated signalcomprises the step of selectively phase modulating said data modulatedsignal at a frequency equal to said predetermined frequency at whichdata is modulated.
 29. A transmission system comprising: a transmitter,including an optical signal source for generating an optical signal ontowhich data is modulated at a predetermined frequency; an amplitudemodulator coupled to the optical signal source for modulating theintensity of said data modulated signal; a clock coupled to theamplitude modulator having a frequency that determines the frequency ofthe amplitude modulator, said frequency of the clock being phase lockedand equal to said predetermined frequency; an optical transmission pathcoupled to said transmitter; and a receiver coupled to the opticaltransmission path.
 30. The transmission system of claim 29 furthercomprising: means for measuring a predetermined characteristic of anoptical signal received by the receiver; means for transmitting thepredetermined characteristic to the transmitter; and means forselectively varying the phase of the amplitude modulation imparted tosaid data modulated signal to optimize the value of the predeterminedcharacteristic.
 31. The transmission system of claim 30 furthercomprising a polarization modulator coupled to said amplitude modulatorand said clock for modulating the state of polarization of the opticalsignal at said predetermined frequency such that an average value of thestate of polarization over a modulation cycle is substantially equal tozero; and
 32. The system of claim 31 wherein said polarization modulatormodulates the state of polarization by tracing the polarization of saidoptical signal along at least a portion of a Poincaré sphere.
 33. Thesystem of claim 31 wherein the polarization modulator modulates thestate of polarization of the optical signal at said predeterminedfrequency with a prescribed phase, and further comprising an electricalvariable-delay line coupling said clock to said polarization modulatorfor selectively varying the prescribed phase.
 34. The system claim 33wherein said electrical variable-delay line is a phase shifter.
 35. Thesystem of claim 31 further comprising an optical phase modulatorcoupling the optical signal source to the amplitude modulator, saidoptical phase modulator providing optical phase modulation to said datamodulated signal while imparting substantially no polarizationmodulation thereto.
 36. The system of claim 35 wherein said clock iscoupled to said optical phase modulator so that said optical phasemodulator provides optical phase modulation at a frequency that is phaselocked and equal to said predetermined frequency.
 37. The system ofclaim 36 further comprising a second electrical variable-delay linecoupling said clock to said optical phase modulator for selectivelyvarying the phase of said optical phase modulation provided by theoptical phase modulator.
 38. The system of claim 37 wherein said secondelectrical variable-delay line is a phase shifter.
 39. The transmissionsystem of claim 38 wherein said predetermined characteristic is thesignal-to-noise ratio of the optical signal received by the receiver.40. The transmission system of claim 39 wherein said predeterminedcharacteristic is the Q-factor of the optical signal received by thereceiver.
 41. The apparatus of claim 1 wherein said amplitude modulationimparts sinusoidal modulation to said data modulated signal.
 42. Theapparatus of claim 14 wherein said amplitude modulator impartssinusoidal modulation to said data modulated signal.
 43. The method ofclaim 25 wherein said amplitude modulation imparts sinusoidal modulationto said data modulated signal.
 44. The apparatus of claim 29 whereinsaid amplitude modulation imparts sinusoidal modulation to said datamodulated signal.